Thyroid hormone (TH) has profound effects on a wide variety of physiological processes in most organ systems, and plays a major role in the maintenance of basal metabolic rate and thermogenesis. TH is also important for the coordinated progression of differentiation and maturation that occurs during development in vertebrate species. The principle secretory product of the thyroid gland is thyroxine (T4), which is converted in peripheral tissues to 3,5,3'-triiodothyronine (T3), the physiologically active form of TH. Thus it follows that the control of intracellular T3 levels by the deiodinases that generate, (types I (D1) and II (D2)) and degrade (type III (D3)) T3 is critical to TH action. However, the physiological roles of the individual deiodinases, in particular D1 and D2, have not been clearly defined. It is generally considered that the D1 functions primarily to generate T3 for export to the plasma, while the D2 activity is thought to generate T3 mainly for local use in tissues. But this concept of separate functions for the two enzymes is based largely on indirect evidence, and is complicated by the fact that some tissues express D1 as well as D2. Experimental proof of the roles of these enzymes has been limited because there are no known specific pharmacological inhibitors of the individual deiodinases. Our general hypothesis is that the D1 and the D2 provide for tissue-specific adaptations of intracellular T3 production in response to endogenous or environmental threats to TH homeostasis. We will test this hypothesis using mouse knock-out (KO) models that are deficient in either D1 or D2 activity or both. In SA number 1 the importance of D2 to TH action, in particular to the feedback regulation of TSH by plasma TH levels, and to the development and function of the CNS will be determined. The D2KO mouse that we have recently developed will be used for these studies. In SA number 2 the physiological role of the D1 in TH economy, in particular its importance in T3 production by the thyroid gland and its contribution to peripheral T3 production, will be assessed. A D1KO mouse will be developed for these studies. In SA number 3 the overall importance of 5'-deiodination in TH action, and whether there is functional overlap in the roles of the D1 and the D2 will be determined. A mouse deficient in both D1 and D2 will be developed by cross-breeding, and the results of studies using this double D1/D2KO model will be compared with those of the D1KO and the D2KO. The thyroidectomized D1/D2KO mouse will also provide an excellent model in which to study the effects of T3 and T4 independently. Taken together these studies will provide unique, previously unattainable insights into the significance of TH metabolic processes under a variety of clinically relevant physiological conditions.